A soldier stands guard near a tank position close to the Russian border near the Ukranian city of Kharkiv March 24, 2014.

U.S. and European security agencies estimate Russia has deployed military and militia units totaling more than 30,000 people along its border with eastern Ukraine, according to U.S. and European sources familiar with official reporting.

The current estimates represent what officials on both sides of the Atlantic describe as a continuing influx of Russian forces along the Ukraine frontier, the sources said.

The 30,000 figure represents a significant increase from a figure of 20,000 Russian troops along the border that was widely reported in U.S. and European media last week.

But U.S. and European security sources noted that these estimates are imprecise. Some estimates put current troop levels as high as 35,000 while others still suggest a level of 25,000, the sources said.

However, the sources said that U.S. and European government experts believe that there has been, and continues to be, a steady and noticeable buildup in the total number of Russian forces along the Ukrainian border, though some military units have rotated in or out of the area.

U.S. and European security sources said that the Russian force deployed along the Ukraine border includes regular military including infantry and armored units and some air support.

Also deployed are militia or special forces units comprised of Russian fighters, wearing uniforms lacking insignia or other identifying markings, similar to the first Russian forces to move into Crimea during Russia's recent military takeover there.

U.S. officials said that what Russian President Vladimir Putin actually plans to do with his forces deployed on the Ukraine border is unknown. Some officials say intelligence information available to policymakers regarding what Putin is thinking, and what he is saying to his advisors and military commanders, is fragmentary to non-existent.

But the portents are potentially ominous. "No one's ruling out the possibility of additional Russian military aggression," one U.S. official said.

U.S. President Barack Obama was non-committal when asked about the 30,000 Russian troops estimate at a news conference in The Hague on Tuesday.

"With respect to the Russian troops that are along the border of Ukraine at the moment, right now they are on Russian soil. And if they stay on Russian soil, we oppose what appears to be an effort of intimidation, but Russia has a right, legally, to have its troops on its own soil. I don't think it's a done deal. And I think that Russia's still making a series of calculations," Obama said.